Contemporary microcontrollers and processors typically contain an inverter-amplifier that is suitable for use as a part of feedback oscillator when it is connected to an external crystal and some other components. A typical crystal oscillator system includes an inverter amplifier with two extra resistances Rf and Rs. Feedback resistance Rf is connected between the input and the output of the inverter/amplifier and provides proper DC biasing and negative feedback. Serial limiting resistance Rs is connected from the output of inverter-amplifier to one of the crystal pins and ensures proper output impedance and power for the crystal oscillator system. The other components of the crystal oscillator system are capacitors CL1 and CL2 connecting the electrodes of the crystal to ground (GND). The values of CL1 and CL2 are determined by the manufacturer's specifications according to the intended use of the crystal. There is a limited choice of component selection to provide specified crystal oscillator characteristics. It is basically limited to the choice of values of Rf and Rs. A desired characteristic of crystal oscillator systems is frequency stability in the specified range of operating voltages, temperatures and variations of parameters of crystal and inverter/amplifier. Another important characteristic of crystal oscillator systems is the ability for fast oscillation start up on power up of supply voltage. Yet another important characteristic of a crystal oscillator system is a drive level compatible with the specified power dissipation in the crystal. Usually crystal manufacturers give the operating voltage drive levels of the crystal in microwatts for specified crystal long and short term stability. Drive levels of the crystal directly effect the stability of the crystal oscillator system: frequency stable crystal oscillator system should have a drive voltage level no more than the specified drive voltage level. Unfortunately the values of Rf and Rs are very difficult to chose to satisfy all main crystal oscillator characteristics: for better frequency stability the value of Rf needs to be small and the value of Rs needs to be high; for reliable oscillation start-up the value of Rf needs to be high and the value of Rs needs to be small. That forces a compromise between the values of Rf and Rs to partly satisfy both frequency stability and reliable start-up of the crystal oscillator and results in a non-optimal operation of the crystal oscillator. The problem is exacerbated for so called “low power” crystal oscillator systems which have very small size and relatively low price compared with so called “high power” crystal oscillator systems. The majority of existing processors have been designed for “high power” crystals which make it more difficult to use them with “low power” crystals.